Implement OAuth 2 support

[alvarosanchez]

Fixes #422
Fixes #428
Fixes #437

[alvarosanchez]

@croudet feel free to review this PR if you want to

[sdelamo]

@alvarosanchez I've verified this works with a Amazon Cognito Oauth 2.0 application with a configuration such as:

micronaut:
  security:
    authentication: idtoken
    oauth2:
      clients:
        cognito:
          client-id: 'xxx'
          client-secret: 'yyy'
          openid:
            issuer: 'https://cognito-idp.us-east-1.amazonaws.com/poolIdChangeMe/'
    endpoints:
      logout:
        get-allowed: true 

I had to override the IdTokenClaimsValidator because the token received by the swagger UI did not include the aud claim but the a claim with the client_id.

I've extracted several constants, if you think it is a bad idea because they make the code harder to read I can revert those changes easily.

[alvarosanchez]

Thanks for the refactor. I didn't add constants because I didn't see them being much used in the project.

I tested myself with Okta, and it worked without any further customisation.